1. Field of the Invention
This invention relates to an image processing apparatus and an image processing method which process image data corresponding to the same region on a printing medium for printing an image in the same region by moving a printing unit (print head) relatively to the same region by a plurality of times.
2. Description of the Related Art
In a serial type inkjet printing apparatus, for the purpose of enhancing evenness of an image to be printed on a printing medium, a multi-pass printing method for performing a print in the same region on the printing medium by a plurality of times of scans by a print head is effectively used. Even in a case of adopting the multi-pass printing method, however, there are some cases where, due to a conveyance error of the printing medium or the like, there occurs a displacement in dot printing position between a preceding printing scan and a following printing scan and an image defect such as density unevenness may be introduced.
FIGS. 1A and 1B are diagrams each explaining the image defect upon performing a multi-pass print of two passes. In the figures, each black circle indicates a dot to be printed in a first printing scan and each white circle indicates a dot to be printed in a second printing scan. Conventionally in the multi-pass printing method, mask patterns having a completing relationship with each other are used to perform a print on the same region in a plurality of times of printing scans by the print head. Therefore, a dot arrangement as shown in FIG. 1A is obtained in the same region on the printing medium.
However, in a case where a dot group printed in the first printing scan is displaced from a dot group printed in the second printing scan due to some cause, the dot arrangement on the printing medium appears as shown in FIG. 1B. That is, the dot group printed in the first printing scan and the dot group printed in the second printing scan overlap with each other, so that blank sheet regions are exposed according to the overlap amount, thus lowering a coverage ratio of dots onto the printing medium, that is, an image density. In addition, when such a printing position displacement is suddenly generated due to a variation in conveyance amount of the printing medium, a speed variation of a carriage, a variation in distance between the printing medium and the print head (distance between a paper and the print head), or the like, there appears a region where the density in the corresponding region alone is low, which is recognized as density unevenness.
Japanese Patent Laid-Open No. 2000-103088 discloses, as a technology of reducing such an image defect, a method in which image data is distributed to correspond to different printing scans at a multi-value stage of the image data before binarization and each of the distributed multi-valued image data is binarized.
FIGS. 2A and 2B are diagrams each showing printing positions of dots upon performing a multi-pass print composed of two passes by using the method disclosed in Japanese Patent Laid-Open No. 2000-103088. In the figures, each black circle indicates a dot to be printed in a first printing scan, each white circle indicates a dot to be printed in a second printing scan, and each hatched circle indicates an overlap dot to be printed as a result of overlap by the first printing scan and the second printing scan. According to the method disclosed in Japanese Patent Laid-Open No. 2000-103088, since the multi-valued data is distributed to correspond to the printing scans, which is thereafter binarized independently from each other, the dot group printed in the first printing scan and the dot group printed in the second printing scan have no completing relationship. That is, there are some pixels where dots are printed in both of the first printing scan and the second printing scan, and on the other hand, there are some pixels where the dot is not printed in any of the first printing scan and the second printing scan.
According to such a dot arrangement, even if the dot group printed in the first printing scan is displaced from the dot group printed in the second printing scan, the coverage ratio of the dots onto the printing medium is not so much varied as shown in FIG. 2B. This is because, there newly appear some regions where dots printed in the first printing scan and dots printed in the second printing scan overlap, and meanwhile, in other regions where two dots are supposed to originally overlap, dots do not overlap.
However, such density variation due to the printing position displacement between the printing scans is not so much noticeable in low-density regions where the number of dots to be printed on the printing medium is originally small. This is because, in the low-density region, dots in both of the dot group printed in the first printing scan and the dot group printed in the second printing scan originally scatter in a small number, and even if these dot groups are displaced more or less, a distance of the dots between the dot groups is not so much small as the dots overlap with each other or are separated from each other. Instead, in the low-density region, there are some cases where, by adopting the method disclosed in Japanese Patent Laid-Open No. 2000-103088, a degree of dispersion of the dots is biased, raising a problem with graininess due to this bias.
For overcoming this problem, Japanese Patent Laid-Open No. 2009-246730 discloses a method in which, multi-valued image data is distributed into plural data corresponding to printing scans, and each of the distributed multi-valued image data is thereafter binarized in such a manner as to be exclusive with each other between the respective printing scans in regard to the low-density region. According to the method in Japanese Patent Laid-Open No. 2009-246730, it is possible to perform a multi-pass print which can restrict the graininess in the low-density region and at the same time, restrict the density unevenness due to the printing position displacement in the intermediate or high-density region.
Incidentally in recent years, under progress of a higher resolution in the printing apparatus, there is adopted a method of printing a plurality of dots in different positions within a pixel region to one pixel after quantization. Japanese Patent Laid-Open No. 2009-246730 also discloses a method in which each of multi-valued data distributed corresponding to two printing scans is quantized into any of 0, 1 or 2 and a dot pattern as shown in FIG. 3 in which the position and the number for printing dots are defined is made to be associated with any one of the levels for binarization. In a case of FIG. 3, for example, in a case where the level is 2 after quantization, one dot is printed in each of the opposing corner positions in 2×2 areas (positions in the top left and bottom right).
However, according to the method disclosed in Japanese Patent Laid-Open No. 2009-246730, since the constant dot pattern as shown in FIG. 3 is used to any of the printing scans, there are some cases where in the intermediate and high-density regions, lack of the density occurs and the density unevenness due to the printing position displacement can not be sufficiently avoided. The reason will be explained as follows.
In general, a size of a dot on the printing medium is designed to be associated with a printing resolution such that, in a case of printing dots on all the pixels corresponding to the printing resolution in the printing apparatus, blank sheet regions on the printing medium are sufficiently covered. As in the case of Japanese Patent Laid-Open No. 2009-246730, in a case where one pixel of the image process is expressed in tone to correspond to 2×2 printing resolution areas, one dot is printed in each of the 2×2 areas (FIG. 4A) to obtain the maximum density. However, in the construction where the multi-value data is distributed and each of the distributed data is then quantized, even if the multi-valued data before the distribution is a signal showing the maximum density (255), the multi-valued data corresponding to each printing scan is reduced to 50% (128). In addition, the multi-valued data is quantized to a tone value (level 2) where two dots respectively are printed in 2×2 areas. As a result, according to the dot pattern in FIG. 3, in the level 2 having the tone value equal in both of the first printing scan and the second printing scan, one dot is printed in each of the opposing corner positions in a region of 2×2 areas (positions in the top left and bottom right). That is, even in a case where a signal indicating the maximum density (255) is inputted in each of all the pixels in the same image region, the dot is printed not in all of the areas, and two dots are printed to overlap in every other area as shown in FIG. 4C. In this case, the number of dots to be printed on 2×2 areas is equal to that in a case where one dot is printed in each of the 2×2 areas, but blank sheet regions remain on the printing medium. That is, the density is lower as compared to a case where one dot is printed in each of the 2×2 areas.
In addition, in this state, when the printing position of the first printing scan is displaced from that of the second printing scan, the printing state of the dots occurs as shown in FIG. 4D, wherein the blank sheet region becomes smaller than that in a case of FIG. 4C. That is, according to the construction in Japanese Patent Laid-Open No. 2009-246730, the graininess in the low-density region can be reduced, but the density lack and the density unevenness in the intermediate and high-density regions can not be sufficiently restricted.